Let’s start by describing the issue. The human body generates a great deal of heat when running. Our bodies are only about 25% efficient, meaning the metabolic processes that keep us alive and keep us moving down the trail (or Shiva forbid- a road) results in 75% of all energy burned being wasted as excess heat. We’re basically a big furnace.

In cold weather, this is a good thing. As long as we fuel the furnace, that generated heat will keep us warm. Even though the outside temperature is lower, our body still has to shed the excess heat. We do this via four mechanisms: Radiation, conduction, convection, and evaporation. All four mechanisms move heat from the hotter human body to the cooler area outside the body.

In hot weather, things get a little dicey. We begin to lose the ability to shed heat, which increases the danger of hyperthermia, which can lead to heat stroke and death. Let’s look at each in detail.

Heat Transfer

Radiation: Radiation occurs when infrared energy is emitted from an object to surrounding objects when no physical contact exists. This is how the sun heats the Earth. Your body sheds some heat via radiation. If you’re wearing any sort of clothing, some of that radiated heat will be reflected back to the body.

Conduction: Conduction occurs when two objects come in contact with each other. The second law of thermodynamics explains how heat is transferred from the warmer to the cooler object until they are the same temperature. This is how a heating pad works. Or why your cold water in your handheld water bottle quickly warms up even on cool days. Our body normally transfers heat to our clothing and surrounding air via conduction. This is also why we can cool down faster when swimming- water is more efficient at transferring heat than air. It’s also why we thaw our Thanksgiving turkey in a sink full of water instead of just leaving it on the counter. This idea plays a role in the mechanism behind the “soaking-wet-shirt phenomenon I’ll discuss later.

Convection: Convection involves the loss of heat through the movement of molecules of air, water, or other such “stuff” across a surface (such as our skin.) The greater the movement of a substance across a surface, the greater the heat loss. My friends in the north are familiar with this idea in the winter when we experience wind chill. This is also why we use fans in warm weather. For our purposes, we just need to know that moving air cools faster than stagnant air.

Evaporation: When a liquid is converted (evaporates) to a gas, heat is lost which produces a cooling effect. This is the purpose of sweat- our body emits the liquid to our skin, then it evaporates into the surrounding air. This mechanical process produces a cooling effect, which in turn cools the skin. For those folks living in the desert, this is the principle behind a swamp cooler- the evaporation of water will cool the air, which can then be circulated to cool a living space.

Bodily Mechanisms

Okay, so those are the mechanisms our body uses to cool itself. Our body employs a few strategies to maximize these processes, including:

Sweating: Our body produces sweat which is evaporated on the skin, thus cooling our skin. Some people seem to believe sweat cools by mechanically eliminating heat from the body, which is false. Sweat cools because it evaporates. We consume water to replace the water lost via sweat to prevent dehydration. In an environment where evaporation is difficult (high humidity), evaporative cooling is less efficient. It’s important to note that sweating has a finite cooling potential. Many runners believe they can combat any heat as long as they drink enough, which is NOT the case.

Vasodialaton: the small capillaries near our skin widen allowing more blood to circulate near the surface of our skin. Our skin is cooled via evaporative cooling from sweat, which cools the blood via radiation, convection, and conduction. The cooler blood is then routed to our body core to help cool our organs… much like the radiator in a car.

Environment

All of these systems work together to allow us to exercise in hot weather. At some point, the environmental conditions will become harsh enough where we begin to lose a few of these cooling options.

For example, when the ambient air temperature is above the temperature of our skin, our body can no longer shed excess heat via via radiation and conduction. In fact, our body absorbs heat from the environment.

Humidity is an issue, too. When the air is saturated with water vapor (100% relative humidity), sweat no longer evaporates, thus eliminating the evaporative cooling effect.

Radiation from the sun is a rather obvious environmental issue, but still worth mentioning. Direct sunlight is going to increase solar gain whereas shade from clouds, trees, or a hat are going to decrease solar gain.

Finally, a lack of air movement can be problematic because we lose the ability to shed heat via convection. A nice breeze will keep you cooler than perfect calm.

Before we get into the discussion of clothing choice, we need to consider the role of insulation. Here in the U.S. we measure this with r-value, which is an expression of a substance or object’s ability to reduce the rate of heat flow. Any clothing will produce an insulation effect by disrupting radiation, conducton, convection, and/or evaporation.

Effects of Clothing

Clothing can either enhance or disrupt each of our body’s cooling mechanisms. Many clothing choices have both pros and cons. Some are dependent on the environment. A few choices may be made based on perceived comfort over actual thermal regulation properties. Needless to say, there’s a lot of personal preferences involved.

All clothing is going to create a radiant barrier that will reflect the radiant heat given off by the body. The lighter the color of the interior of the material, the more pronounced the effect. Think of those silver foil emergency blankets- they work because they reflect a great deal of radiant heat (they also greatly reduce convection because they’re non-porous.) Clothing can also reflect radiant heat from the sun. A hat can shade the upper body, for example.

Speaking of convection- any clothing that prevents air from penetrating will greatly reduce convective cooling. No air flow = no convective cooling. This is usually the goal of outer shells for cold weather activities. In addition to convective cooling, fabrics that prevent airflow also prevent sweat from evaporating which further reduces their cooling potential.

The more air a fabric traps, the greater the r-value (the more it prevents heat transfer.) This is usually why thicker fabrics keep you warmer. When a fabric becomes saturated with sweat (or water), the water is much better at transferring heat than the trapped air. As a result, saturated clothing won’t keep you as warm (or will keep you cooler depending on your goal) as dry fabrics.

It all seems pretty logical, doesn’t it? How about this riddle- the goat-herding Bedouins of the Sinai Peninsula live in a desert where temps routinely top 100 degrees F. Their preferred clothing?

Thick black robes.

At first it seems to defy logic because black absorbs solar radiation from the sun and the thick robe inhibits conduction, convection, and evaporation, right?

Wrong.

Since it’s a loose-fitting robe, air can flow freely inside. The sun heats up the fabric, which heats the air inside. Hotter air rises which pulls cooler air from the bottom as the hottest air escapes the top. This airflow promotes convective and evaporative cooling, which keeps them cool.

If you understand the science of heat transfer, the mechanisms the body uses to cool itself, and how various clothing affects both, you can begin to make educated clothing choices.

So What Do We Wear When Running to Keep Us Cool?

Should we all run in thick black robes?

Probably not. The Bedouins aren’t running around. They’re not producing as much heat as a runner would, so there’s less excess to shed.

A better option is to use a method that utilizes as many principles discussed above as possible.

Naked (or minimally-clothed) is among the best options. If there’s no clothing to interfere with radiation, convection, conduction, or evaporation, we should be able to stay cooler longer.

We may still outrun our cooling capacity. In that case, we have to stop generating as much heat by slowing down, stopping, and/or utilizing an external cooling mechanism like sitting in a cool stream.

Some clothing items may help, like a wide-brimmed hat in sunny environments. It will provide shade for our head and upper torso which reduces solar gain. A straw hat that allows airflow would be ideal.

If clothing must be worn, loose-fitting options would be ideal because they allow air to pass (promoting evaporative and conductive cooling.)

Fabric type does matter… which is the crux of my anti-tech shirt rant. In hot, dry conditions, a tech shirt mechanically moves sweat from the skin to the surface of the shirt. In theory, the sweat would then evaporate from the surface of the shirt which cools the shirt (evaporative cooling) then cools the skin via conductive cooling (heat moves from hotter skin to the cooler shirt.)

There’s two significant problems with this theory. First, sweat evaporating on the skin is going to have more “cooling potential” than sweat evaporating off the shirt then cooling the skin via conduction. Second, the shirt fabric itself acts as an insulator to heat transfer because it traps air.

These problems disappear if the tech shirt is hydrophllic (the fabric absorbs water) and becomes saturated. The wet shirt clings to the skin. Since water is a better conductor of heat than air, the cooling on the surface of the shirt cools the underlying skin much more efficiently. It’s not as good as naked skin, but a lot better than a non-saturated shirt.

Materials like cotton, which become saturated much easier, will keep you cooler because this effect occurs sooner. Unfortunately, clingy, drenched clothing is uncomfortable and usually chafe underlying skin easier. Many people prefer tech shirts because they stay drier. That’s an entirely valid reason to use them… as long as the other principles are understood.

So What Should You Use?

Experiment. Test out a variety of fabrics. Figure out what conditions require specific types of clothing. Don’t disregard bare skin. In the event you’re overheating in hot weather, consider the effects of your clothing choice. If you’re interfering with even one of your body’s natural mechanisms, you may be handcuffing your cooling capacity.

Also remember your body can only cool itself so much. When the ambient air temperature rises above your skin temperature, you lose a lot of cooling potential. Same deal with high humidity. Drinking more water won’t help because you’re already sweating at capacity. Walk a bit. Sit in the shade. Place an ice-filled bandana around your neck. Train for heat acclimation.

A thorough understanding the science behind heat transfer and thermoregulation helps immensely. It helps you become a better consumer of products that supposedly “keep you cool.” It also helps you improve your self-experimentation abilities.

Good luck and stay cool!

###

Like the idea of self-experimentation and running? You’d probably love the Squirrel Wipe book! Check it out here!

Yep – your understanding is a bit off. A layer of water / sweat will inhibit cooling by convection, because it really is a better insulator than a dry shirt. But this is in conditions when the humidity is so high that the sweat’s not evaporating. A soggy shirt, in that case, is much warmer than a dry one. Those people in humid climates that change into a dry shirt to “cool off” aren’t idiots – it really does help if the sweat isn’t able to evaporate.

Rob Y

May 29, 2013

Yep. Common practice running in the Deep South is to frequently wring that sweat soaked shirt out and put it back on. Feels great! Also will take the shirt off and soak it in a cold creek (when possible), wring it out and put it back on. Nice cooling effect.

Michael

May 21, 2013

Only wool for me, comfy when wet, warm when it needs to be, cools you off nicely. Ahh, who’d have thunk natural animal hair would work better than an oil byproduct or a plant?

Rob Y

May 22, 2013

Wool is definitely my clothing of choice during the cold months. Retains and regulates your body heat quite well even when soaking wet (our winters hear are typically just above freezing but with rain). Never considered using wool in the summer, will have to give that a shot as well. Just need the UV protection is all.

Michael

May 22, 2013

I like it in summer, except on the really dry 40+C days when sweat dries too quickly to soak the skin/shirt. But nothing really helps then apart from tipping water on yourself.

Rob Y

May 20, 2013

I’ve used both the loose Sun Precautions type UV sun fabric clothing as well as more form fitting Patagonia sun shirts with UV fabric. Found the each was about equally effective. Can definitely tell a difference between shirts with and without UV fabric. Wouldn’t even think of doing a summer ultra not in UV clothing. I’m convinced after many trials and experiments of my own.

Rob Y

May 20, 2013

Great article. A lot of good information here. Unfortunately I have to disagree with your anti-tech shirt rant. The important thing to remember and you do point this out is that not all tech shirts are the same because they all use different fabrics. Unfortunately most people’s poor tech shirt experiences I believe stem from the fact that they’re wearing race event provided tech shirts. I know from first hand experience as an RD that these shirts are expensive and so often it’s the cheapest types of tech shirts that are bought by RDs and provided to race participants. Yes, this cheap stuff sucks! Basically like running in a plastic bag that chafes! Really treat these shirts as you would your everyday cotton T, but don’t run in them.

It pays to do your homework. There are several bands with very superior fabrics that really do provide a measurable effect on cooling. I speak from several first hand experiences I’ve had running in the extreme heat and humidity to know that I definitely felt much cooler in these fabrics than I did in other lesser quality fabrics or even going shirtless.

The other consideration is protection from solar radiation. While you can regularly apply sunscreen during a long run or race it can be quite annoying to do so and you get the gunk everywhere! Several of the modern tech fabrics have SPF ratings equal to sunscreen and don’t lose there properties even when saturated wet. That’s another HUGE piece in why I prefer UV protective clothing over going shirtless. I noticed immediately out at Badwater that wearing this type of clothing made me feel much cooler than when I was walking around in a normal cotton t shirt. Huge difference.

So quality of tech fabric is key. Do your research before purchasing. Yes some of this stuff is expensive but it also lasts so much longer than the cheap stuff and the cooling and UV protection are priceless!

Rob, Was the UV clothing at badwater the looser fitting super vented kind? If so the cooling effect may have been due to venting better than a cotton T-shirt. Looser tech fabrics also really don’t wick because they aren’t in contact with the skin so you would still have the benefit of evaporative cooling. Basically, looser ventilated clothing would have a similar bedouin effect to what Jason described above.

Just a thought,
Brad

Matt

May 20, 2013

Great article, learned quite a bit about the science behind cooling… Thanks!

Shane

May 20, 2013

What effect does sunscreen have on cooling? As you note some of the tech shirts are not comfortable when they are wet and frankly cotton isn’t either.